Stabilization of ZnS nanoparticles by polymeric matrices: syntheses, optical properties and recent applications

Abstract

ZnS has been one of the most investigated II–IV semiconductor materials, known for very interesting optoelectronic properties and its versatile applications in various fields. The transition from bulk to nano regime has brought forth some drastic changes in the optical properties of ZnS. In this review article, we have comprehensively discussed the surface passivation of ZnS nanoparticles by various polymeric ligands/matrix. Systematic investigation has been reported for state of the art synthesis and novel strategies for fabrication of ZnS nanocomposites have been discussed. A brief account of the pioneer works for ZnS has been described and their fundamental properties have been discussed. The characterization techniques in understanding the formation and stabilization of ZnS nanocomposites are highlighted. The article primarily focuses on the main features, synthesis techniques, and optical properties of polymer based ZnS nanocomposites. We have tried to cover recent application and developments in field of polymer based ZnS nanocomposites in biosensing, cell tagging, optoelectronic devices, heterogeneous catalysis, photocatalytic application etc. Overall this article provides an insight to optimize the synthetic routes and condition for better utilization of ZnS nanocomposites and provide an outlook to expand these applications to other areas, such as drug delivery, labeling, tracking agents, bioanalytical sensors, fluorescent probes, optics, information storage, and optoelectronics.

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Ashish Tiwari

Ashish Tiwari received his Ph. D degree from Guru Ghasidas Vishwavidhyalaya: A Central University, Bilaspur, India in 2012. The topic of his PhD dissertation was Luminescence properties of ZnS nanophosphors using different capping molecules. He is currently working as an Assistant Professor in Department of Chemistry, in Government Lahiri College Chirmiri, India. He is recipient of Young Scientist Award from Chhattisgarh Council of Science & Technology (CCOST) in 2012. His current research interest is synthesis of luminescent nanomaterials.

S. J. Dhoble

S. J. Dhoble obtained his Ph.D. degree from RTM Nagpur University, Nagpur, India in 1992. He is presently working as an Associate Professor in Department of Physics, R.T.M. Nagpur University, Nagpur, India. He has published more than 500 articles. He serves as an editor of Luminescence: The Journal of Biological and Chemical Luminescence, John Wiley & Sons Ltd. Publication. He is recipient of Prof. B.T.Deshmukh Research Award 2016 and Prof. B.P.Chandra Research Award 2016, Outstanding Scientist-2015, form Venus International Foundation and Advanced Materials Scientist Letter Awards-2011 for outstanding contribution in Advanced Materials Letter. His current research interests are synthesis of solid state lighting nanomaterials and radiation dosimetry phosphors.

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1. Introduction

Nanoscaling the size of materials has been one of the most recent advances in the field of science. Nanomaterials are being continuously explored for practical applications in electronic, chemical, biotechnology industries and biomedical fields.¹ It has revolutionized understanding of mankind towards matter. It facilitates material scientist to fabricate materials atom by atom or molecule by molecule.² This technology exploits the novel phenomenon and the property of matter at nanoscale. The reduction in the size of these nanomaterials provides drastic improvement in the characteristic properties resulting due to quantum size effect.³ These factors can change significantly properties such as reactivity, strength and electrical characteristics. The growth and catalytic chemical reactions occur at surfaces and hence, a given mass of material in nano particle form will be much more reactive than the same mass of material made up of larger particles. The surface-to-volume ratio for nanoparticles is exceedingly large, typically million-fold times as compared to bulk. As a result, most of the properties of such nanoparticles (including optical) becomes extremely sensitive due to surface characteristics. It is also possible to manipulate the surface to influence and control the properties of nanoparticles. Many of these unique properties are extremely promising for emerging technological applications, such as nanoelectronics, nanophotonics, biomedicine, information storage, communication, energy conversion, catalysis, environmental protection, and space exploration.^4–7

Nanocomposites are new generation novel and hybrid materials consisting of multiphase solid material. The essential condition is that atleast one of the component (phase) is in the nanometric range. Nanocomposites can include three dimensional metal matrix composites, lamellar composites, colloids, porous materials, gels, copolymers in which nanosized material are dispersed with the bulk matrix. The morphology can be different such as platelets, fibers, spheroids etc. The composite possess novel and collective performances which original components lack. The properties of the nanocomposites depend on the individual components, morphology and the interface characteristic. The improved and enhanced characteristics of nanocomposites such as super hydrophilicity, super hydrophobicity, thermal and chemical stability, improved mechanical property, electrical conductivity, abrasion resistance makes them worthy material for application in engineering purposes.^8,9

ZnS quantum dots dispersed in a polymer matrix or other solid bulk materials, which has evolved as a new class of probes in biochemical analysis, can be successfully applied to the ultrasensitive detection of proteins, DNA sequencing and their conformational changes, clinical diagnostics, immunoassays, luminescence tagging, cellular imaging, as well as drug delivery and biosensing systems.¹⁰

This concept article does not claim to serve as an exhaustive review of all polymer based ZnS nanocomposites reported till date, but we tried to highlight key features involved in the formation of ZnS nanocrystals dispersed in polymeric matrix. Important examples of research in the field of ZnS nanoparticles stabilized by polymeric ligands are highlighted. The focus is primarily emphasized on the optical studies, synthesis, recent studies on the improvement of their novel properties and potential applications of ZnS nanocomposites to various branches of science and technology.

2. Types of nanocomposites

Nanocomposite materials can be categorized on the basis of their matrix materials^11–13 as ceramic matrix nanocomposites (CMNC), metal matrix nanocomposites (MMNC) and polymer matrix nanocomposites (PMNC). The ceramic matrix nanocomposites (CMNC), mainly comprises of the Al₂O₃/SiC system. The incorporation of high strength nanofibers into ceramic matrices result in novel and advanced nanocomposites having high toughness and superior failure characteristics. Metal matrix nanocomposites (MMNC) consist of a ductile metal or alloy matrix in which a material having nanosize regime is implanted. These materials behave as metal as well as ceramic. They have ductility and toughness with high strength and modulus. This makes them suitable for production of materials with high strength in shear/compression processes and high service temperature capabilities. Out of ceramic matrix nanocomposites (CMNC) and metal matrix nanocomposites (MMNC), polymer matrix nanocomposites (PMNC), the PMNCs are more suitable for industrial applications due to their light weight and ductility. However, they exhibit poor thermal, mechanical, electrical and other properties as compared to CMNC and MMNC. These properties can be dramatically improved by reinforcing the matrix with nanomaterials. Nanosized reinforcement (such as particles, whiskers, fibres, nanotubes etc.) with polymer matrix results in enhanced magnetic, electronic, optical or catalytic properties.¹⁴ The commonly employed fillers in PMNC are nanoclays, Carbon nanotubes CNT's, grapheme, metallic and semiconductor nanoparticles such as ZnS, CdS etc. Polymers are considered as good choice as host materials because of their ability to be modified, thus, yielding a variety of bulk physical properties. They are characterized by having long term stability and flexible reprocessability. Hence, these can have novel properties such as fluorescence, electroluminescence and optical nonlinearity.¹⁵ During this process the nanoparticles grow in the polymer matrix, whose molecules play a dual role; first stabilize and isolate the generated nanoparticles avoiding aggregation and after drying, serve as a confined medium that protects the nanoparticles surface. As nanoparticles and biomolecules are of a similar length scale, it seems logical that the combination of biomacromolecules to nanomaterials can provide interesting tools for mimicking the biomolecules which are present at cellular systems, probing the mechanisms of biological processes, as well as developing chemical means by handling and manipulating biological components. A schematic illustration of a nanocomposite stabilized by various groups that might be present in a polymer is represented in Fig. 1.

Fig. 1 Schematic representation of nanocomposite material showing surface passivation and modification of the nanoparticles for linking polymer/macromolecules.

3. ZnS: a potential candidate

Confinement of size has made ZnS (a class II–IV semiconductor nanoparticles) an attractive and promising material because of their size dependent optoelectronic and luminescent properties which can be easily tuned. When the size of semiconductor particles is reduced to nanometer scale their physical properties differ significantly from bulk counterparts. The nanoscaling results in quantum confinement of the photogenerated electron–hole pair. When the radius of the particle approaches the Bohr radius of the exciton, the energy gap increases, consequently there is a blueshift of the optical absorption edge with respect to the bulk material. These characteristics make it a potential candidate in preparation of semiconductor quantum dots and their application in electronics, optoelectronics, magnetic and non-linear optics.

Zinc sulfide have wide range of applications in solar cells, infrared window materials, photodiode, cathode-ray tube, and multiplayer dielectric filters light-emitting diodes (LEDs), electroluminescence, flat panel displays, infrared windows, sensors, lasers, and biodevices. It occurs in two crystalline forms (allotropes), one in the hexagonal system (forms at high temperature) and other in cubic system (low temperature stable polymorph). The mineral in hexagonal system is called as wurtzite, whereas the cubic system is known as sphalerite or zinc blende.

3.1 Zinc blende (sphalerite)

The mineral sphalerite is the chief ore of zinc consisting mainly of zinc sulphide in crystalline form, but invariably contains impurities of iron. It crystallizes in the cubic crystal system. Zinc sulphide has 4 : 4 arrangements in which zinc and sulphide ions are tetrahedrally coordinated, as shown in Fig. 2.¹⁶ The lattice constant for zinc sulfide ions in the zinc blende crystal structure is 0.4 nm, calculated from geometry and has ionic radius of 0.074 nm (Zn²⁺) and 0.184 nm (S²⁻). It forms cubic close packing (ccp) of sulphide ions having ABCABCA… arrangement of layers.¹⁷

Fig. 2 Zinc blende crystal structure and tetrahedral arrangement. Adapted from ref. 16.

3.2 Wurtzite

Wurtzite is another form of zinc sulfide, less frequently encountered than sphalerite. This form also has 4 : 4 arrangements with zinc and sulphide ions tetrahedrally surrounded having a coordination number 4.¹⁸ The sulphide ions form hexagonal close packing (hcp) with zinc ions¹⁹ in every alternate tetrahedral void giving rise to ABABA… arrangement (Fig. 3).

Fig. 3 The hexagonal wurtzite crystal structure. Adapted from ref. 19.

Wurtzite is a very attractive material for optical application especially in nanocrystalline form. In recent years, the optical properties of impurity doped nanocrystals have attracted much attention. The doping of impurities in nanostructures modifies the electronic states as well as electromagnetic fields and hence, the optical properties may change drastically in nanoscale.

4. Role of the dopant ion

Doping quantum dots with activator ions (a transition metal ion or rare earth) have several advantages over undoped quantum dots, such as high thermal and photochemical stability, large Stoke shifts leading to avoidance of self-absorption, and high excited state lifetime. Doped semiconductor nanocrystals are important because of both high luminescence efficiencies and lifetime shortening.²⁰ Yang et al. have done an extensive study on doped and co-doped ZnS nanocrystals and reported a red-shift of emission wavelength in the range from 500 to 550 nm upon doping with Mn²⁺, Cu²⁺ and Pb²⁺.²¹ Peng et al. have observed two blue luminescence bands centered at 411 and 455 nm in undoped ZnS. They also identified a third green luminescence band centered at 500 nm in Cu²⁺ doped ZnS.²²

These properties are due to strong exchange coupling between localized moments of paramagnetic dopant and band electrons of the semiconductor. Dopant impurities play an important role in changing the electronic structure and transition probabilities of the host material.²³ As the electron clouds of localized ions are allowed to overlap and hybridize, the excitation energy in doped nanocrystals can be rapidly transferred into energy state of the impurity, and then followed by an efficient and fast radiative recombination with emission. The essential criterion for the dopant to effectively show luminescence properties depends on the fact that only a small fraction of initial dopant added is incorporated into the crystal lattice and a large proportion of remaining impurity ions resides on the surface or form impurity precipitates. Hence location of dopant ions in semiconductor dots is important as it affects optical properties. It should be located at substitution site rather than interstitial site. The identical ionic states and compatible size of dopant with respect to host favors incorporation of dopant at metal sites.

5. Approaches for preparing nanocomposites

The fabrication and synthesis of functional nanostructured materials have applications in diverse fields such as biomedicine, optics, and green energy production. The synthesis of such materials requires techniques that afford precise control over material properties. Surface charges as well as presence of capping molecules can modify the emission spectra of nanoparticles. Surface functionalized nanoparticles can be attached to larger particles.²⁴ Surface passivation can be achieved in several ways. It can be done by suspension of nanocolloids in a liquid, forming in nanocrystals a matrix, nanocrystals formation in cage as zeolite, capping of nanocrystals with passivating molecule. These polymers can not only encapsulate nanoparticles for better exploitation of their characteristic properties but also can modify the surface and/or to control the growth of nanoparticles. Surface modification of quantum dots using biocompatible polymer matrix has several advantages such as: (i) passivation of surface defects results enhanced fluorescence and photostability (ii) providing it biocompatibility with respect to toxic core of QDs (iii) provide functional groups for further bioconjugation with suitable ligands. Polymers can be a better option as they have good mechanical and optical properties conferring high kinetic stability on nanometre-sized semiconductor particles. Nanocomposites containing inorganic quantum dots dispersed in a polymer matrix exhibit increased photogeneration efficiency, broadened spectrally tunable photoresponse and enhanced carrier mobility.²⁵ The techniques used for synthesis procedures are in situ polymerization, melt intercalation, template synthesis and sol–gel method. Amongst the strategies used to synthesize organic–inorganic hybrid nanocomposites, two approaches have been developed. In the first route inorganic domains are being incorporated into a polymer matrix by a sol–gel technique. The second method involves incorporation of pre-synthesized inorganic colloidal nanoparticles into a polymer matrix. The in situ precipitation method has been an efficient technique to manipulate and to process nanoparticles in technologically useful formulations based on nanocomposites.

6. Different routes for synthesizing polymer based ZnS nanocomposites

The controlling of size precisely depends on nucleation and growth processes. The use of biopolymers inhibits/modifies the crystallization process and hence, careful monitoring of this parameter is a biomimetic strategy that influences the nanocluster growth.²⁶ Cohen et al. proposed that nanoclusters are stabilized by interactions between nanocluster surface and polymer ligands depending upon the functional group present in the polymer microdomains.²⁷ Method used for generation of ZnS nanocomposites can be categorized as (i) intercalation of nanoparticles with the polymer or pre-polymer from solution (ii) in situ intercalative polymerization; melt intercalation (iv) template synthesis (vi) in situ polymerization; (vi) sol–gel process. The most common strategy in in situ technique is to synthesis functionalized nanoparticles as a sol or dispersion and a monomer or resin is added and subsequently polymerized.²⁸ Functionalization of polymers is a necessary step to avoid aggregation of the nanoparticles on the large surface area of the polymer. Attachment of the end functionalized polymers to semiconductors nanoparticles improves interaction between them. Precise control on the size and improvement of the surface structure can be achieved by this method. The in situ polymerization techniques are mostly employed for polymer nanocomposite synthesis as larger amount of nanoparticles can be anchored to the polymer surface. The chemical structure of the polymer governs the effective interaction between the nanoparticles and polymer. Hence, hyperbranched polymers or dendrimers with high density of functional groups are effectively used for synthesizing semiconductor-polymer nanocomposite.²⁹

6.1 Template synthesis of ZnS nanocomposites

Zhang & Yang synthesized zinc sulfide (ZnS) nanoparticles by deposition on the surface of natural halloysite nanotubes (HNTs) as template to produce ZnS/HNTs nanocomposites.³⁰ The results indicated that ZnS nanoparticles were uniformly attached on the surface of HNTs with narrow particle size distribution centered at ∼10 nm, and were prevented from aggregation by HNTs with more exposed active sites. It showed excellent photocatalytic activity for the degradation of eosin B under UV light, better than pure ZnS and HNTs. Microwave assisted synthesis of g-poly(acrylamide)/ZnS (CPAZ) nanocomposite was done by Gupta et al.³¹ In this report chitosan was used as template for grafting of acrylamide onto it. The size of the CPAZ nanocomposite particles was found in the range of 19–26 nm as suggested by TEM studies the stability of the CPAZ was confirmed by electrical studies with respect to zeta potential. The prepared nanocomposite was found to be antimicrobial against Escherichia coli bacteria and was also investigated for drug release behavior. Sharma et al. described controllable continuous method to produce porous and hollow ZnS nano- and microspheres, as shown in Fig. 4 (ref. 32) using poly(ethylene glycol) methyl ether (PEG), polyvinylpyrrolidone (PVP), ethylene oxide/propylene oxide block copolymer (Pluronic F-38), and cetyltrimethylammonium bromide (CTAB) as templates to synthesize ZnS nanocomposites in a spray pyrolysis process. Wang et al. employed a biomimic method using polysaccharide as template, to synthesize ZnS quantum dots doped with lanthanide ions.³³ According to the results of TEM and absorbance, nanocrystals with an average size of 6 nm were formed under mild condition without any toxic and expensive agent applied. PL studies revealed that PL intensity became more prominent with increasing lanthanide doping concentration, with optimum concentration upto 1%. Campos et al. reported a nanocomposite of ZnS:Mn quantum dots and a third generation PAMAM-OH dendrimer (ZnS:Mn@PAMAM-OHG = 3) as template.³⁴ The structure of ZnS:Mn was not changed after coupling with PAMAM-OH, which was evidenced by the analysis of the emission spectra of the compounds. ZnS nanoparticles (NPs) was synthesized by Mamiyev & Balayeva using a facile chemical route with a narrow size distribution in the MA/octene-1 copolymer matrix as template.³⁵

Fig. 4 Morphology of the different ZnS hollow nanospheres showing different stages of particle formation from dense spherical shell to hollow spheres.³² Adapted with permission from ref. 32. Copyright 2015 American Chemical Society.

Du et al. prepared ZnS nanoparticles (NPs) prepared inside nanosized colloidal mesoporous silica (CMS) by in situ generation procedure.³⁶ The ZnS-containing CMS (ZnS-CMS) was modified with 8-hydroxyquinoline (HQ) by the coordination with zinc atom of ZnS surface. The size of the generated ZnS NPs in the channels of CMS was estimated to be about 2.5 nm. The existence of ZnS inside the CMS hosts resulted in a decrease in the surface area and pore volume. The maximum excitation and emission wavelengths of the polymer nanocomposites had a red-shift with the increasing content of HQ–ZnS-CMS was accounted on the concept of cooperative interaction between the HQ–ZnS-CMS and polymer matrix with blue emission. He et al. immobilized ZnS NPs on the surface of poly (vinylidene difluoride) (PVDF) mixed with methacrylic acid (MAA)-trifluoroethyl acrylate (TFA) copolymer electrospun nanofibers.³⁷ Zinc ions were introduced onto the surface of nanofibers by coordinating with the carboxyls of MAA, and then sulfide ions were added to react with zinc ions to form ZnS particles under hydrothermal condition. Wang et al. reported a new template membrane, acrylic acid grafted microporous polypropylene (PP) membrane, prepared using the supercritical (SC) CO₂ as a solvent.³⁸ Subsequently zinc ions were anchored in the membrane by forming a monodentate complex with the carboxyl groups in the template membrane which on treatment with Na₂S aqueous solution formed ZnS nanoparticles within the PP matrix. Khiew et al. synthesised ZnS nanostructured materials in the micellar solution system, containing chitosan laurate as the surfactant.³⁹ They found that surfactant molecules in water solution self-assembled to form unique architecture. It can be adopted as the reaction template for the formation of nanomaterials. The size of the resulting nanoparticles is greatly affected by the surfactant concentration and range from 2 to 10 nm. Yang et al. reported a facile synthesis of spherical ZnS nanocrystals modified with dianion of 3-ferrocenyl-2-crotonic acid (FCA) from one-dimensional (1-D) coordination polymer. Zn(II) coordination polymer so formed acted as both precursor and template.⁴⁰ Moreover, an ionic liquid 1-butyl-3-methylimidaz-olium tetrafluoroborate (BMIMBF₄) solution system worked as an assisting template in the experiment. The morphology of the synthesized nanocrystals is spherical and with diameters ranging from 40 to 60 nm. Hu et al. 2011 reported that graphene oxide nanosheet provides a two dimensional growth template for ZnS and can act as a good dispersant for stabilizing the nanocomposites.⁴¹

6.2 Sol gel synthesis of ZnS nanocomposite

In sol–gel process, first a colloidal suspension (sol) is formed and subsequently gelation of the sol occurs to form a network in a continuous liquid phase (gel). The sol–gel process synthesis is a multistep process in which hydrolysis and condensation occurs sequentially. Precursors can be either metals alkoxides or inorganic and organic salts. The silica polymer hybrid materials are the most studied class of sol–gel prepared inorganic–organic hybrids.⁴² Mu et al. described a method in which no 3- (mercaptopropyl)trimethoxysilane (MPS) was required as coupling agent and acid or alkali as catalyst to synthesize silica nanospheres on the ZnS cores.⁴³ We synthesized Cu doped ZnS nanoparticles inside the pore of an inorganic silica gel matrix by modified Stober sol gel method and discussed advantages of silica coating over other capping agents, which are used for surface passivation of nanoparticles.⁴⁴ Silica glasses containing both ZnS quantum dots (QDs) and luminescent lanthanide ions were synthesized by Planelles-Aragó et al. using one pot sol–gel method.⁴⁵ They prepared Eu³⁺-doped and Eu³⁺, Mn²⁺-codoped ZnS nanocrystals dispersed in a transparent silica matrix. Polycrystalline semiconductor nanocrystals having an average size of 5–6 nm were obtained at low temperature. The luminescent interactions between ZnS QDs, Eu³⁺ and Mn²⁺ ions provided information about the organization of different species in the nanocomposite. The study described the parameters vital for designing photonic materials.

6.3 In situ polymerization synthesis of ZnS nanocomposite

In synthesis of polymer nanocomposites, polymer scaffold can be utilized as a matrix for ordered and anisotropic arrangement of nanoparticles. In the in situ formation of nanoparticles in polymer matrices, the pre-synthesized nanoparticles are integrated into polymer matrix and thus, has inherent advantage of uniform distribution of nanoparticles in polymer.⁴⁶

Transparent mercaptoethanol capped ZnS/polymer bulk nanocomposites with high particle contents were prepared via γ-ray irradiation initiated polymerization by Lu et al. In this article, authors have extensively discussed strategies involved in the design and tailoring of the surface of the nanoparticles, choice of the monomer and the selection of the polymerization route.⁴⁷ Gao et al. functionalized ME capped ZnS NPs with 5-amino-1,10-phenanthroline (APhen) by a ligand-exchange process and synthesized a series of APhen-functionalized ZnS nanoparticles (NPs)-polymer transparent nanocomposites with tunable fluorescent emission via in situ bulk polymerization.⁴⁸ Lu and Yang have extensively discussed the general design principles and different fabrication approaches of high refractive index inorganic nanoscale building blocks into processable, transparent organic matrices.⁴⁹ Lu et al. synthesized functionalized 5-(2-methacryloylethyloxymethyl)-8-quinolinol (MQ) MQ–ZnS NPs by a facile ligand exchange approach and subsequently incorporated the nanoparticles into the polymer matrix by in situ bulk polymerization. The nanocomposites was found to be of good thermal stability and high transparency.⁵⁰ Liu et al. synthesized PTU/ZnS nanocomposites by integrating the pre-made thiol capped ZnS NPs into the polymeric monomer which was subsequently polymerized to form nanocomposites. The thiol capped ZnS nanoparticles with a diameter of about 5 nm were fabricated into the molecular chains of PTU via the formed covalent bonds between the capped ZnS and the matrix. The investigations demonstrated that ZnS nanoparticles were uniformly dispersed in the PTU matrix even at high contents. The nanocomposites had high refractive index and transmittance.⁵¹ Guo et al. reported the controllable synthesis of ZnS nanocrystal-polymer transparent hybrids by using polymethylmethacrylate (PMMA) as a polymer matrix using graft polymerization method.⁵² Synthesis of ZnS NC–PMMA nanocomposite hybrids. The ZnS NC–PMMA nanocomposite hybrid was synthesized via free radical polymerization in situ by one step, with AIBN as initiator, ZnS NCs, MMA. The functionalization of the hydroxyl-ending alkyl group introduced onto the surface of ZnS NCs enhances their dispersity in solvent, allowing the particle size of NCs to be controlled. Bai et al. developed an efficient one-pot strategy for the preparation of hydrophilic amine-functionalized nanocomposites by using hydrophobic fluorescence quantum dots ZnS:Mn²⁺@allyl mercaptan (QDs@AM) as building blocks through novel light-induced in situ polymerization.⁵³

Size control of as-prepared hydrophilic nanocomposites was tuned by varying the concentrations of the monomers. These nanocomposites reported are the first to be utilized for the facile, highly sensitive, and selective detection of nitroaromatics. The novel surface modification method developed offers a general strategy for fabricating hydrophobic nanocomposites with hydrophilic properties. Mattoussi et al. reported the specific and strong binding of polyhistidinecontaining proteins to CdSe/ZnS nanoparticles with a dihydrolipid acid (DHLA) ligand layer without NTA or bivalent ions present.⁵⁴ Recently, it was shown that the polyhistidine moiety can directly bind to the inorganic particle, apparently to Zn atoms present in the ZnS shell, as demonstrated by control experiments with different target molecules and ligands without NTA or free carboxylic groups.⁵⁵ Anderson and Chan evaluated the impact of amphiphilic polymer composition on the size, transfer efficiency, and biocompatibility of tri-n-octylphosphine oxide/hexadecylamine-stabilized semiconductor ZnS-capped CdSe and CdS-capped CdTe_xSe_1−x quantum dots (QDs).⁵⁶ The adsorptions of various proteins onto the surface of these QDs were investigated and the effect of surface chemistry on non-specific protein binding indicates that they will have implications in the design of QDs and other nanoparticles for biological and biomedical applications. Shahi et al. studied structural parameter and grain sizes for ZnS nanoparticles in polyvinyl alcohol (PVA) matrix by using synchrotron radiation.⁵⁷ Blue shift in the optical band gap of ZnS nanoparticles with increasing PVA concentration was observed and accounted due to the effect of quantum confinement of electrons. Dark and photocurrent decreases with increasing PVA concentration, showed nonlinear behavior for I–V plots.

Mohan et al. reported the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA).⁵⁸ The QDs were synthesized via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The formation of core–shell structure was confirmed by Raman spectroscopy. The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. Mohan et al. synthesized, ZnS : Ni²⁺ nanostructures through chemical precipitation method using poly(2-hydroxyethyl methacrylate) (pHEMA) as capping agent. The structural, morphological and optical properties were found to be different for various concentration of pHEMA. The surface morphological analysis reveals that the pHEMA capped nanoparticles showed homogeneous smooth surface.⁵⁹

6.4 Melt intercalation synthesis of ZnS nanocomposite

Melt mixing is basically an industrial technique. As no solvents are employed in this process, this can be applied to polymer processing industry to produce nanocomposites based on usual compounding devices, such as, extruders or mixers. In this method, during the extrusion, clay is mixed with the molten polymer, whose chains penetrate inside the galleries of clay layers inducing its intercalation.⁶⁰ The melt compounding is most economically attractive and environmentally viable since it can be performed using scalable melt extrusion. However, the dispersion is particularly difficult because of the high viscosity of the polymer and the low bulk density of the nanoparticles, which makes feeding into extruders very difficult. Kharapapong et al. reported the intercalation of ZnS into the interlayer spaces of montmorillonite by solid–solid reactions and in situ formation between Zn(II)-montmorillonite and Na₂S molecule at room temperature.⁶¹ The melt intercalation method has limited practical application and hence, very few studies have been done.

We tried to compare the dependence of particle size on synthesis strategy as shown in Table 1. It can be seen that almost same particle size is obtained in in situ chemical methods.

Table 1 Dependence of particle size on the synthesis method

Nanocomposite structure	Particle size (nm)	Synthesis method	Ref.
ZnS PVP (poly vinylpyrrolidone)	XRD – 2.18	Chemical method in aqueous medium	62
	TEM – —
ZnS polyvinyl alcohol (PVA)	XRD – 3.6	Ion exchange reaction	63
	TEM – 2.8
ZnS@acrylated 2-(2-mercapto-acetoxy)-ethyl ester (AMAEE) hyperbranched polyester, Boltorn_H2O	XRD – 2.7	Chemical method	64
	TEM – 1–4
ZnS TiO2	XRD – 2.7	Reverse micelles	65
	TEM – 3–4
ZnS-films sodium carboxymethyl cellulose	XRD – 1.8–2	In situ precipitation	66
	TEM – 3.0
ZnS:Mn SiO2	—	Stober method (sol gel)	67
ZnS graphene	XRD – —	Solvothermal	68
	TEM – 1–2
ZnS:Mn PEG polyethylene glycol	XRD – —	Hydrothermal	69
	TEM – 3.0
ZnS thin films o-phenylene diamine (o-PDA)	XRD – 2.7	Spin coating	70
	TEM – 3.3
ZnS:Cu nanofibers PVA	XRD – —	Electrospinning	82
	TEM – 3.0
ZnS nanoparticles mesoporous silica CMI-1	XRD – 3.4	Incipient wetness method	71
	TEM – —
ZnS-nanocomposites montmorillonite	XRD – —	Hydrothermal	72
	TEM – 25

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7. Functionalization of polymeric matrix and interaction with ZnS nanoparticles

The proper surface functionalization of nanoparticles is prerequisite for any synthesis as it determines interaction of nanoparticles with the environment and colloidal stability of the particles. The chemical structure of the polymeric ligand/stabilizing molecules facilitate the nanoparticles to bind on their surface, thereby, preventing the growth of particles during synthesis, which otherwise may lead to aggregation of the nanoparticles. The particles do not cling to each other due to forces such as electrostatic repulsion, steric exclusion or a hydration layer on the surface. The stabilizing molecules are attracted to the surface of the nanoparticles through chemisorption, electrostatic attraction or hydrophobic interaction, due to functionalities present on the terminal end of the stabilizer. The polarity of the solvent that is used to disperse nanoparticles plays an important role in preventing the particles to agglomerate. The polar groups present in the passivating molecules solubilizes in polar solvent while the hydrophobic ends of the molecules solubilize in non-polar solvents. Amphiphilic groups present in the molecules (such as polyethylene glycol PEG etc.) help in dispersion of the particles in variety of solvents with intermediate polarity. The most common functionalities present in the polymers that stabilizes the nanocomposites such as thiols, amines, phosphines under goes a change in stereochemical arrangement which is a dynamic binding and unbinding processes. This might cause the attached ligand to detach from the composite consequently leading to agglomeration.⁷³ Fig. 5 shows the different interactions between the nanoparticles surface and the passivating molecule.⁷⁴ Generally, in order to avoid ZnS nanoparticles agglomeration, synthetic polymers were adopted as the stabilizing or supporting materials to keep the size of ZnS in the nano-range. Probably the first example of QDs protected with polysaccharides was reported by Chen et al.⁷⁵ This group prepared CdSe–ZnS quantum dots protected with carboxymethyldextran and polylysine, and they proved the high affinity of the QDs towards the glucose binding protein-concanavalin A (Con A). We recently reported the facile synthesis of hydroxyl functionalized polymer based ZnS nanocomposites⁷⁶ and have shown that large number of hydroxyl groups in the polymer matrix facilitates the complexation of metal ions (Fig. 6). Guo et al. modified ZnS NCs with MPS, which subsequently allowed methacrylate groups to attach onto the surface of NCs⁵² hence the NCs incorporated into PMMA matrix via free radical polymerization (Fig. 7).

Fig. 5 Modular design of surface ligands for biocompatible nanomaterials. (a) Anchor links capping molecules to the crystallite through hydrophobic interaction with native ligands or through atoms that bind directly to the nanocrystal surface. (b) Stabilizer region incorporates polyethylene glycol (PEG) or zwitterionic chains that strongly bind water molecules to impart hydrophilicity and prevent nonspecific adsorption. (c) Capping molecules may feature reactive groups serving as a tether point for covalent conjugation to biofunctional molecules after ligand exchange. (d) The biofunctional units can offer targeting, therapeutic or sensing capability.⁷⁴ Reproduced with permission from ref. 74. Copyright 2016 Nature Publishing group.

Fig. 6 Schematic representation of the proposed interaction between (a) ZnS–PVA, (b) ZnS–starch, (c) ZnS–HPMC nanocomposites and (d) ZnS–polymer nanocomposites.⁷⁶ Reproduced with permission from ref. 76. Copyright 2016 John Wiley & Sons, Ltd.

Fig. 7 Synthesis of hydroxyl-coated ZnS NCs and ZnS/PMMA nanocomposite hybrids. The hydroxyl-ending alkyl group introduced onto the surface of ZnS NCs enhances their dispersity in solvent and MPS functionalize ZnS contain double bonds that facilitate nanocomposite hybrids via free radical polymerization in situ.⁵² Reproduced with permission from ref. 52. Copyright 2007 Springer-Verlag.

ZnS/bacterial cellulose/epoxy resin (ZnS/BC/E56) nanocomposites having good transparency and flexibility were very recently prepared by Guan et al.⁷⁷ They reported that ZnS nanoparticles were smaller than 50 nm, and homogeneous distribution within the composites were obtained when Zn²⁺ concentrations were not more than 1 wt%. Ummartyotin et al. used (PVP) as the stabilizer for forming the precursor of ZnS.⁷⁸ They reported that PVP by virtue of the carbonylic functionality can offer exposure of the ZnS nanoparticles to the outer environment, leading to a very hydrophilic surface which makes the ZnS stable in water. Amide within pyrrolidone structure gives water soluble properties. PVP acts as a stabilizing agent for coupling to amino functionalized colloids and OH surface coating minimizes non-specific cellular binding.⁷⁹

8. Factors affecting the size and morphology of ZnS nanocomposites as revealed by different characterization techniques

The chemistry of nanoparticles synthesis is governed by several factors and therefore, their shape and size strongly depends on the reaction conditions like pH, temperature concentration of precursors, methods of synthesis etc. From Fig. 8, it can be seen that crystallinity of the ZnS nanoparticles depends on the concentration of the polymer. If the wt% of the polymer matrix is increased the amorphous character of the nanocomposites is increased and in XRD diffraction pattern peak broadening is observed. Bhaishya and Sarkar studied the effect of change the molar ratio of the constituent concentration on ZnS–PVA nanocomposite formation.⁸⁰ Spherical grains and decrease in size was reported as the molar ratio of ZnS:PVA decreases. PL intensity was found to increase as the molar ratio decrease. The precise control of particle size depends on two important factors i.e. nucleation and growth. Nanoparticle formation takes place when rate of nucleation is greater than rate of growth of the particles. The factors that determine nucleation are degree of super saturation and low solubility of the substances while growth rate depends on amount of precursors, diffusion rate of material from solution to the growing particles, ease of orientation of molecules from solution into solid lattice, synthesis temperature etc. Any, adhered impurity or any adsorbed species on surface of particle can also be a growth inhibiting factor. NPs synthesized at higher super saturation exhibit high photoluminescence quantum efficiency. The particle size of ZnS NPs could be tuned by controlling molar ratio of the reagents and polymer. Mbese et al. reported that prepared nanocomposite showed a reasonably good interaction between metal sulphide nanoparticles and PMMA matrix.⁸¹ The PMMA acted as a good host matrix, since it did not affect the shape and properties of metal sulphide nanoparticles dispersed to it as shown in Fig. 9. Wang et al. reported that morphology of the Cu-doped ZnS/PVA nanofibers was smooth and uniform.⁸² They were longer than several millimeters, with diameters about 300 nm and without the presence of any beads (Fig. 10).

Fig. 8 Dependence of XRD on the concentration of polymer. Broadening of peaks with increasing GR content suggest loss of crystallinity and increase in the amorphous character.¹⁵¹ Reproduced with permission from ref. 151. Copyright 2010 Royal Society of Chemistry.

Fig. 9 SEM images of ZnS, Cds and HgS/poly(methyl methacrylate) nanocomposites. PMMA act as a good host matrix as it retains the shape of the metal sulphide NPs.⁸¹ Reproduced with permission from ref. 81.

Fig. 10 SEM images of 2.5% Cu-doped Zn(Ac)₂/PVA nanofibers (a) and 2.5% Cu-doped ZnS/PVA nanofibers. The morphology of the fibers is smooth, uniform having several millimeters length and diameters about 300 nm.⁸² Reproduced with permission from ref. 82. Copyright 2006 Elsevier B.V.

It can be seen from Fig. 11 that different morphologies of ZnS-related 1D nanostructures, such as nanowires, nanobelts, nanotubes, nanocombs, nanoawls, can be synthesized by altering the reaction conditions.⁸³ Jayasree et al. synthesised mannosylated chitosan–ZnS nanocomposites. The SEM image (Fig. 12) showed a homogeneous distribution of nearly monodisperse particles with spherical morphology, having size of ∼150 nm. Particle size of the bioconjugated nanocrystals deduced from AFM also measured to be ∼140 nm, which correlated well with the hydrodynamic diameter measured using DLS.⁸⁴ Hollow nanospheres of ZnS nanospheres semiconductor (50–70 nm) were synthesized by Ma et al. in aqueous solutions of a triblock copolymer (P123) at room temperature⁸⁵ (Fig. 13).

Fig. 11 Different morphologies of 1D-ZnS nanostructures such as nanowires, nanobelts, nanotubes, nanocombs, nanoawls.⁸³ Reproduced with permission from ref. 83. Copyright 2011 Elsevier B.V.

Fig. 12 Representative (A) SEM and (B) AFM images of mannosylated chitosan-ZnS NCs. It has a homogeneous distribution of nearly monodisperse particles with spherical morphology, having size of ∼150 nm.⁸⁴ Reproduced with permission from ref. 84. Copyright 2011 Elsevier B.V.

Fig. 13 TEM (a) and HRTEM (b) micrographs of hollow ZnS nanospheres in the presence of P123. [P123] = 20 g L⁻¹. The inset shows the corresponding electron diffraction pattern.⁸⁵ Adapted with permission from ref. 85. Copyright 2003 American Chemical Society.

9. Absorption spectra of ZnS nanocomposites

UV-Vis spectroscopy is one of the important tools to study the optical property of polymer based ZnS nanocomposites. It helps to understand the interaction between matrix and the nanofiller, and analyses the role of nanofillers in enhancing the properties of the nanocomposites. The tuning of the desired optical properties can be achieved by careful analysis of the absorption characteristic of the nanocomposites. Lowering of the light scattering occurs when the samples are passivated by the polymer matrix. The absorption intensity at the initial and the last wavelength nearly equals to zero. The sharpness of an absorption peak is believed to due to a high monodispersity of the semiconductor nanoparticles as observed by Kumbhojkar et al.⁸⁶ The optical studies revealed that absorption edge of PVP polymer shifted from 322 nm to 305 nm as PVP concentration increased in PVP capped ZnS:Mn nanocomposites, PL spectra was found to be strongly affected by PVP concentration.⁸⁷

It is clear from Fig. 14 that the samples exhibit absorption edges which are blue shifted with decreasing particle size as the concentration of zinc ion precursor increased. This blue shift of the absorption edges for nanocrystals arises from quantum confinement effect in the nanoparticles. The absorption peaks of all the doped samples shift with respect to the dopant ion (Fig. 15) and hence, the particle size and band gap also changed. The absorption peaks of all samples were blue shifted compared to 345 nm.⁸⁸ Kim et al. studied the effect of concentration of sodium polyphosphate (SPP) on position of UV absorption edge. As the amount of SPP increased from 0.0 to 1.89 g, there was a remarkable blue-shift phenomenon (Fig. 16). Moreover they reported a change in spectrum shape as the input amount varied, and it was thought that there exists a threshold injection level where SPP begins to act efficiently. It is the typical blue-shift phenomenon of absorption edge commonly observed for very fine particles.⁸⁹ Chae et al. synthesized highly loaded ZnS nanoparticle (NP) array within organically functionalized (HS–CH₂CH₂CH₂–S≡) mesoporous MCM-41 (FM-41) channels via repetitive insertion of ZnS reverse micelles.⁹⁰ It was observed that during the first loading of the ZnS NPs into the FM-41 host (ZnS@FM-1), an absorption shoulder appeared near 275 nm which was further enhanced by further loading of ZnS NPs into the host (designated as ZnS@FM-2). It clearly indicates that enhancement of the exciton absorption increased with the loading amount of nanoparticles. This enhancement in intensity might be attributed to more confinement of ZnS NPs existing within the host channels. Lu synthesized ZnS/polythiourethane (PTU) nanocomposites via immobilization of thiophenol (PhSH)/mercaptoethanol (ME)-capped ZnS nanoparticles into a PTU matrix.⁹¹ The absorption onset appeared at 310 nm, which obviously was blue shifted due to quantum confinement of the ZnS nanoparticles within the polymer matrix. Lu et al.⁹⁸ synthesized thiol capped ZnS NPs dispersed in poly urethane-methacrylate macromer (PMMU) and found that optical transmittance of nanocomposite films is above 95% at 550 nm even when the content of thiol-capped ZnS nanoparticles were increased from 0 to 86 wt%. The authors suggested that small sized ZnS nanoparticles uniformly distribute in the films and the films have a good optical homogeneity. Ghosh et al.⁹² synthesized PVP-encapsulated ZnS nanoparticles and examined the optical properties of these ZnS nanoparticles with varying ageing time at the reaction temperature, concentrations of PVP and S²⁻ ions. The absorption edges were reported at 335 nm and 323 nm for ZnS uncapped, ZnS–PVP nanocomposites, respectively. Narrow size distributions of the synthesized NPs were clearly indicated by sharp absorption edges in the absorption spectra.

Fig. 14 Absorption spectra of ZnS quantum dots. The band gap value increased with concentration of polymer.⁶³ Reproduced with permission from ref. 63. Copyright 2013 Elsevier B.V.

Fig. 15 UV-Vis spectra of undoped (a) and Mn²⁺ (b), Co²⁺ (c), Ni²⁺ (d), Cu²⁺ (e), Ag²⁺ (f) and Cd²⁺ (g) doped ZnS nanoparticles. The absorption peaks of all samples were blue shifted compared to 345 nm (3.6 eV) of bulk ZnS.⁸³ Reproduced with permission from ref. 83. Copyright 2012 Elsevier B.V.

Fig. 16 (a) UV absorption spectra for ZnS:Cu nanocrystals capped with SPP 0–1.51 g. Spectrum shape changes as the input amount of SPP varies (b) position of absorption edge plotted as a function of SPP input amount.⁸⁹ Reproduced with permission from ref. 89. Copyright 2006 Elsevier B.V.

10. XPS studies of ZnS nanocomposites

Spectroscopy is an inevitable tool of characterizing polymer nanocomposites. Various spectroscopic techniques unveil the nature of dispersion of nanoparticles within the matrix polymer, the interactions between filler and polymer, functionalization of the polymer or nanofiller, interlayer spacing within the filler platelets, mechanical properties of the nanocomposites, etc.⁹³ X-ray photoelectron spectroscopy is a surface chemical analytical technique. XPS can be used to imprint change in chemical structure of polymer by adding ZnS nanoparticles. It helps to establish any charge transfer interaction between semiconductor particle and polymer component. XPS studies provides useful information in obtaining the elemental composition of the top 0–10 nm surface, in detecting elements contaminating the surface, provides information regarding the local bonding of atom, in identification of chemical states for the elements present in the sample etc. It also helps in revealing nature of the interfaces between nanoparticles and polymer matrix.⁹⁴ In the XPS studies of ZnS–GO by Pan and Liu, it was observed that a strong interaction between Zn ion and the carbonyl oxygen prevented the nanoparticles from aggregating within the polymer matrix.⁹⁵ Most carbons were in the form of sp² bonds, and the intensity of oxygenated functional groups (O=C–OH, C–O–C and C–OH) on carbon sheets in ZnS–graphene was decreased compared with that of GO. This implies that GO has been reduced to graphene^93,94 (Fig. 17 and 18).

Fig. 17 XPS images of ZnS–graphene. Nanocomposites.⁹⁶ Reproduced with permission from ref. 96. Copyright 2016 Royal Society of Chemistry.

Fig. 18 XPS images of RGO–ZnS nanocomposites.⁹⁷ Reproduced with permission from ref. 97. Copyright 2014 Royal Society of Chemistry.

Lu et al. synthesized novel ZnS–poly(urethane-methacrylate macromer) (PUMM) nanocomposite films with high refractive index by incorporating thiophenol (PhSH)–4-thiomethyl styrene(TMSt)-capped ZnS nanoparticles into a urethane-methacrylate macromer (UMM), followed by spin-coating and ultraviolet radiation initiated free radical polymerization. The XPS depth profiling technique demonstrated that the ZnS nanoparticles were also dispersed homogeneously in the depth scales of the polymer matrix.⁹⁸

Small and Johnston, reported that X-ray photoelectron spectroscopy (XPS) spectra showed shifts in the 2p electrons of Zn and S in both doped ZnS nanocrystals and in cellulose fibres coated with doped ZnS nanocrystals⁹⁹ (Fig. 19). In some cases, the shift was very large (up to 1.4 eV). This indicates that there is a strong bonding between nanocrystals and cellulose fibre.

Fig. 19 XPS spectra for 2p electrons in ZnS:Mn nanocrystals and ZnS:Mn coated cellulose fibres. Zn 2p above and S 2p below.⁹⁹ Reproduced with permission from ref. 99. Copyright 2016 Elsevier B.V.

11. Effect of polymer matrix in modifying PL studies of ZnS nanocomposites

The surface properties of nanostructured materials have significant effects on their electronic/optical properties owing to the fact that a large portion of atoms are located at or near the surface of the nanoparticles. Surface atoms usually have unsaturated or dangling bonds, and the heterostructural interfaces also contain strain-induced defects. These defects can induce extra electronic states within the bandgap, which act as electron/hole trapping centers. It can result in potential quenching of the photoluminescence (PL) and charge transport.¹⁰⁰ Dynamic growth processes play an important role in defining the surface structure of NPs and hence, the PL properties. Interpretation of PL evolution behavior during nucleation and growth of NPs is, therefore, essential in order to elucidate the role of surface state and surface passivation in PL improvement and photostability.¹⁰¹ The study of photoluminescence (PL) provides important information relating to different energy states available between valence band and conduction band responsible for radiative recombination. Steady-state photoluminescence is widely used to investigate the efficiency of charge carrier trapping, migration and charge transfer. It also helps in understanding the behaviour of electron–hole pairs in nanoparticles. Vishwakarma & Vishwakarma¹⁰² explained in their article that when the radius R of the crystallites is smaller than ∼2 exciton Bohr radii, electrons and holes can be considered as two confined particles bound by an enforced coulomb interaction and when crystallite radius is larger than ∼4 exciton radii, the ground exciton is treated as a rigid sphere, confined as a quasiparticle. In between these two limiting cases both the electron and hole confinement and their coulomb interaction are considered. In case of nanocrystals, the electron, holes and exciton have limited space to move and their motion is possible for definite values of energy. As a result, the continuum of states in the conduction and valance band are broken down into discrete states with energy spacing relative to band edge, which is inversely proportional to the square of the particle radius resulting in the widening of the band gap as compared to the bulk. Quantum dots or nanoclusters exhibit discrete electron energy levels with high oscillator strength and strong luminescence. As the optical properties are strongly dependent on particle size, a particle size distribution is expected to cause inhomogeneous broadening of optical spectra. The PL spectra often exhibit well defined peaks associated with band-edge luminescence and recombination at defects. These are also broadened inhomogeneously due to particle size distribution.

Polymer based nanocomposites particularly of ZnS has gathered a great scientific attention due to its unique properties. These nanocomposites have better advantages in comparison to intrinsically photoluminescent polymer as they have low quantum efficiency and poor stability due to oxidation of the surface. The role of polymer matrix in altering the surface states of the ZnS nanocomposites may be monitored by photoluminescence studies. PL properties of pure samples and nanocomposites may differ because it is generally sensitive to the synthetic conditions, crystal size, and shape. Furthermore, compared to pure ZnS nanoparticles, the nanocomposite particles become larger after the coating so as to result in a red shift of emission band. When PL intensity decreases after dispersion of ZnS with a polymer matrix it might indicate that there is a high probability of electron–hole recombination in nanocomposites.¹⁰³

Li et al. studied Ni/ZnS core/shell nanocomposites¹⁰⁴ and reported that there was a decrease in the fluorescent intensity of nanocomposite as compared to the pure ZnS, this significant decrease indicated that the presence of Ni nanoparticles in nanocomposite strongly reduces the photoluminescence intensity (Fig. 20).

Fig. 20 PL spectra of the samples: ZnS sample (a) and Ni/ZnS nanocomposites (b). A small red shift in the emission intensity can be seen.¹⁰⁴ Reproduced with permission from ref. 104. Copyright 2009 Elsevier B.V.

The PL spectra having a strong and narrow excitonic emission with a low deep level emission suggest that the synthesized nanocomposites are of excellent optical quality having few defects. These characteristics make it a good candidate for high-efficiency UV optoelectronic applications.¹⁰⁵ ZnS nanoparticles were prepared on the surface of polyacrylonitrile (PAN) and methyl methacrylate (MMA)/butyl methacrylate (BMA)/acrylic acid (AA) copolymer nanofibers by Zhou et al. The band position of the photoluminescence spectrum of the ZnS/PAN and MMA–BMA–AA nanocomposites had an 80 nm blueshift in comparison with that of the corresponding bulk ZnS sample.¹⁰⁶

Generalova et al. studied the effect of photo-activation of semiconductor nanocrystals result in the enhancement of luminescence intensity and also there is a blue-shift in emission wavelength (Fig. 21). The intensity of fluorescence emission (the so called fast photoluminescence in which the prompt emission takes place within 1 μS after excitation) was twofold enhanced as compared to the fluorescence intensity of free QDs under the same conditions before embedding into ethyl cellulose (EC) particles.¹⁰⁷ It can be attributed to the fact that the hydrophobic polymer caused surface reconstruction of the surface shell of nanocrystals. Possible traps in QDs are, as a rule, surface atoms which must be optimally constructed and passivated with some polymers to get rid of traps.^108,109 The fluorescent EC particles were successfully attached with antibodies this open a new door for its further utilization in various diagnostic tests. Klaush, et al. reported that for ZnS:Cu/polymer nanocomposites the luminescence properties of the particles were retained but a shift of the excitation band to higher wavelength was observed¹¹⁰ (Fig. 22). This shift can be explained by an overlapping absorption band of the added octylamine which was used for hydrophobic surface functionalization thus allowing it for the integration into polymers. In another approach, CNT-ZnS:Mn luminescent nanocomposites were prepared by precipitating semiconductor NPs on carbon nanotube's surface followed by electrophoretic deposition on Al substrate. It was found that the electrophoretic characteristics (EPD) weight depostion, current density and deposition rate and PL studies clearly indicates that surface modification of by PVP and EG significantly modify the EPD properties and PL intensities.¹¹¹ Beltran Hurac et al.¹¹² studied the effect of the magnetically ordered Sr-doped lanthanum manganite (LSMO) on the optical properties of ZnS:Mn. They reported a marked suppression of the PL intensity of the 598 nm peak as the content of LSMO increases (as shown in Fig. 23a), and was attributed to the interaction between the self-trapped excitons (located on MnO₆ octahedra) and the surface defect (vacancy) states of LSMO with the d-electron states of ZnS:Mn. PL intensity of LSMO/ZnS:Mn at 1 : 3 wt% decreased when the temperature increased from 8 to 300 K (Fig. 23b). This decrease in PL intensity can be described in terms of the carrier transfer process from donor states to Mn³⁺ ions in LSMO. Ghosh et al. were probably the first to synthesise ZnS/Dendrimer nanocomposites with amino-, carboxyl- or hydroxyl-terminated Polyamidoamine (PAMAM) dendrimer.¹¹³ Average size of ZnS nanoparticles within the dendrimer matrix was found to be in the range of 2.2–3.1 nm. ZnS nanoparticles with high monodispersity and photoluminescence efficiency were obtained by tuning various experimental parameters. An eight-fold increase in super-saturation of initial reactants caused a sharp focusing of size distribution by 38% and photoluminescence quantum efficiency also increased from 1.3% to 4.2%. It was demonstrated that pH is a critical determinant of aggregation in PAMAM dendrimer, which in turn strongly influences nanoparticle stability against flocculation. The authors described that strong electrostatic interaction between Zn²⁺ and –NH₂ or –COOH groups controls the growth and stabilization of the ZnS NPs within the dendrimer matrix and –COOH and –NH₂ terminated dendrimer provide better protection than –OH-terminated dendrimer. The bright photoluminescence of ZnS NPs are obtained with –COOH or –NH₂-terminated dendrimer due to better coordinating ability of these groups as compared to –OH group The study revealed that photoluminescence and surface charge of these nanocomposites are governed by the surface functionality of the dendrimer molecule. The cathadoluminescence intensity improved immensely compared to pure ZnS and it also showed better field emission (Fig. 24) as reported by Jha et al.¹¹⁴

Fig. 21 Fluorescence intensity of the hydrophobic QDs with λ_em = 546 nm in water–ethanol media (1) and EC containing embedded hydrophobic QDs (2). The intensity of the nanocomposites were twice higher than that of the native nanocrystals.¹⁰⁶ Reproduced with permission from ref. 106. Copyright 2009 Elsevier B.V.

Fig. 22 Comparison of fluorescence emission before and after phase transfer with octylamine (slit: 5 nm; gate: 5 ms). A shift of the excitation band to higher wavelength was observed.¹¹⁰ Reproduced with permission from ref. 110. Copyright 2010 Elsevier B.V.

Fig. 23 (a) PLspectra of ZnS:Mn and LSMO/ZnS:Mn nanocomposites at 1 : 1, 1 : 3, 1 : 5 and 1 : 10 wt% and at 300K (b) PL spectra of LSMO/ZnS:Mn at 1 : 3 wt% at 8 and 300 K. Insets show the optical images of the ZnS:Mn (left) and LSMO/ZnS:Mn (right) when exposed to 325 nm light. There is a moderate suppression in the PL intensity (∼20%) along with a weak red-shift (∼10 nm) of the 598 nm peak.¹¹² Reproduced with permission from ref. 112.

Fig. 24 Cathadoluminescence intensity of the Pure ZnS and ZnS–CNF composite material.¹¹⁴ Reproduced with permission from ref. 114. Copyright 2015 Royal Society of Chemistry.

12. Application of ZnS nanocomposites

ZnS nanocomposites are attracting considerable interest as viable multifunctional and biomedical materials and research into them is growing due to their unique physical and chemical properties.^115,116 Some of the important areas of application are listed as follows.

12.1 Photocatalyst

Chen et al. reported very recently a simple solid-state method for synthesis of ZnS/graphene nanocomposites.¹¹⁷ The results indicated that nanocomposites exhibited superior photocatalytic activity as compared to pure ZnS, owing to the reduction of photoinduced electron–hole pair recombination induced by the introduction of graphene. The photocatalytic performances of products were evaluated by methyl orange. It was found that the nanocomposite obtained by the reduction of NaBH₄ exhibited higher photoactivity as compared to that obtained by the direct addition of graphene. This can be attributed to the high specific surface area and the enhanced synergetic effect between ZnS and graphene. Pourahmad, reported synthesis of ZnS/MCM-41 nanocomposite.¹¹⁸ The photocatalytic activity was evaluated using basic blue 9 or methylene blue (MB) as model pollutant under UV light irradiation. The effect of ZnS, MCM-41 support and different wt% of ZnS over the support on the photocatalytic degradation and influence of parameters such as ZnS loading, catalyst amount, pH and initial concentration of dye on degradation were evaluated. Bin et al. prepared novel graphene/ordered mesoporous necklace-like ZnS nanocomposite (GR–ZnS). It had enhanced photocatalytic performance with 97.5% decomposition of methyl orange (MO) after 30 min under UV-light irradiation and can find potential applications in water purification.¹¹⁹ Graphene nanosheets–zinc sulfide (GNS–ZnS) nanocomposites assisted by microwave irradiation were synthesized by Hu et al.¹²⁰ It showed an excellent photocatalytic activity towards the photodegradation of methylene blue. A one-step hydrothermal method was used to synthesize a novel nanocomposite, SnO₂/ZnS by Hu et al. for photocatalytic degradation of refractory dye Rhodamine B (RhB) under simulated and natural sunlight.¹²¹

12.2 Optoelectronics and device fabrication

The applications of nanocomposites are extremely broad, ranging from solid-state amplifier films to transparent magnets. Nanocomposite structures have resulted in transparent materials with unusually high RI, magnetic properties, and excellent mechanical properties. It has been extensively discussed by Beecroft and Ober.¹²² Synthesis of 2-(2-hydroxyphenyl)-benzoxazol (BOX) functionalized ZnS nanoparticles (ZnS-BOX NPs) with blue-light emission have been reported by Liu et al.¹²³ The ZnS-BOX NPs were added into the polymer matrix by in situ bulk polymerization and had a blue-emission at 429 nm. The near-white light-emitting nanocomposites with low-cost chemical components can be potentially used to fabricate novel photoelectric devices. Nanosized ZnS were synthesized in the conducting polyaniline matrix by Dutta and De. The wavelength of optical absorption peak of ZnS nanoparticle increases from 270 to 330 nm with the decrease of polyaniline concentration.¹²⁴ Studies on direct current (DC) electrical conductivity as a function of temperature suggest that three-dimensional Mott's hopping process occurs in ZnS–polyaniline nanocomposites. The correlated barrier hopping is confirmed from temperature dependent alternating current (AC) conductivity. The incorporation of ZnS nanoparticles enhances the barrier height. Piret et al. reported a new optoelectronic composite with a highly organized mesoporous silica core and a uniform ZnS shell (200 nm in thickness).¹²⁵ The optical properties of the new nanocomposite have been evaluated by photoluminescent spectroscopy. Large size aggregates in ZnS were found and hence, no quantum size effect could be observed. Cheng et al. dispersed ZnS NPs to prepare polymer nanocomposites with tunable refractive index by varying the nanostructures content in the matrix.¹²⁶ They have investigated optical, dynamic and thermomechanical properties of nanocomposites formed of 2-mercaptoethanol (ME)-capped ZnS NPs in polymer matrix obtained by free radical initiated polymerization. The nanocomposites exhibited optical transparency in the visible range, thermal stability and good mechanical properties. The optical transparency of the nanocomposites and the dependence of the refractive indices on NP loading indicate that the prepared nanocomposites can be potentially applied for fabricating optical devices. Sang et al.¹²⁷ synthesized a new single probe, polyethyleneimine (PEI)/Mn–ZnS nanocomposite, for two-color imaging and three-dimensional sensing. The two PL bands of the nanocomposite obtained at 495 nm and 585 respectively were orthogonal, and hence, allowed the discrimination of eight proteins just by recording the three-channel optical signals of the single probe. A series of high refractive index (RI) ZnS/PVP/PDMAA hydrogel nanocomposites containing ZnS nanoparticles (NPs) were successfully synthesized via a simple ultraviolet-light-initiated free radical co-polymerization method and was recently reported by Zhang et al.¹²⁸ The NPs were found to be well dispersed and stabilized in the PVP/PDMAA hydrogel matrix up to a high content of 60 wt% in the hydrogel nanocomposites. The equilibrium water content of ZnS/PVP/PDMAA hydrogel nanocomposites varied from 66.8 to 82.0 wt%, while the content of mercaptoethanol-capped ZnS NPs correspondingly varied from 30 to 60 wt%. The resulting nanocomposites are clear and transparent and their RIs were measured to be as high as 1.58–1.70 and 1.38–1.46 in the dry and hydrated states, respectively, which can be tuned by varying the ZnS NPs content. In vitro cytotoxicity assays suggested that the introduction of ZnS NPs added little cytotoxicity to the PVP/PDMAA hydrogel and all the hydrogel nanocomposites exhibited minimal cytotoxicity towards common cells. The hydrogel nanocomposites implanted in rabbit's eyes can be well tolerated over 3 weeks. Hence, the high RI ZnS/PVP/PDMAA hydrogel nanocomposites with adjustable RIs developed in this work might be a potential material for artificial corneal implants. Qin et al. described a facile microwave-assisted synthesis of ZnS nanoparticles embedded in reduced graphene oxide (RGO).¹²⁹ The material was applied as anode materials of sodium-ion batteries (SIBs) and exhibit excellent sodium storage properties. A series of transparent, highly fluorescent, organic–inorganic nanocomposite films were prepared by Shi et al. in which mercaptoethanol-capped ZnS nanoparticles were incorporated into a copolymer of trialkoxysilane-capped poly(MMA-co-Hq–CH₂–HEMA).¹³⁰ The hybrid nanocomposites had good optical transparency in the visible region. The nanocomposites that contained the ZnS nanoparticles were stable and displayed high fluorescence emission at 500 nm. ZnS/PVA nanocomposites for nonlinear optical applications were very recently studied by Ozga et al.¹³¹ This work can open a new stage for operation by photovoltaic features of the well known semiconductors within a wide range of magnitudes. ZnS quantum dots (QDs) of different sizes were synthesized by a simple chemical co-precipitation method at room temperature, by varying pH value of the reaction mixture by Kole et al.¹³² The authors claims that for the first time, size dependent nonlinear optical property, such as second harmonic generation (SHG) of 1064 nm Nd:YAG laser fundamental radiation was applied to synthesize ZnS QDs using the standard Kurtz–Perry powder method. In order to study the possibility of the synthesized ZnS QDs in different device applications, ZnS/PMMA (polymethylmethacrylate) nanocomposites were also synthesized. Monodispersed and well passivated ZnS semiconductor nanoparticles with average size of 3.4 nm were prepared in situ in chitosan film by Wang et al. The novel biomacromolecule/QDs nanocomposite film has large third-order optical nonlinear absorption.¹³³

12.3 Treatment of effluents

Vitor et al.¹³⁴ synthesised zinc sulphide nanoparticles and ZnS/TiO₂ nanocomposite by utilising excess of biologically generated sulphide, with the treated effluent of acid mine drainage. This work can have wider application by coupling an Acid Mine Drainage (AMD) bioremediation system with the synthesis of metal sulphide nanoparticles and nanocomposites.

12.4 Energy storage

Polymer nanocomposites have gained significant research interests because of their promising potential for versatile applications ranging from environmental remediation, energy storage, electromagnetic (EM) absorption, sensing and actuation, transportation and safety, defense systems, information industry, to novel catalysts, etc. The most recent advances in polymer nanocomposites for energy storage (i.e., electrochemical capacitors and batteries), energy saving (i.e., electrochromic devices and carbon dioxide capture), and anticorrosion (conductive and non-conductive polymer nanocomposite anticorrosive coatings) applications has been recently reviewed by Yang et al.¹³⁵ Ramachandran et al. 2015 solvothermally synthesized zinc sulfide decorated graphene. They reported the application of these nanocomposites as supercapacitor and for energy storage.¹³⁶ Nanosized ZnS particles on the 2D platform of a graphene oxide (GO) sheet was synthesized by Zhang et al.¹³⁷ ZnS–GR nanocomposites acts as an organic dye-like macromolecular “photosensitizer” for ZnS rather than an electron reservoir and the concept can be utilized for designing GR-based composite photocatalysts for targeting applications in solar energy conversion.

Recent strategies used to realize the modifications of metal chalcogenide based nanocomposites have been extensively reviewed by Gao et al.¹³⁸ for advanced energy conversion and storage (ECS) devices (including fuel cells, photoelectrochemical water splitting cells, solar cells, Li-ion batteries and supercapacitors).

12.5 Sensors

Wang et al. synthesized Chitosan/zinc sulfide (CS/ZnS) nano-composite films by simulating bio-mineralization process the hydrothermal stability and fluorescence properties of the films were studied. They reported that nanocomposite has the sensing properties to detect lead ions.¹³⁹ The study revealed that fluorescence emission of the nano-composite films has ZnS particles having dimension of less than 20 nm. The fluorescence emission (363 nm) of the nano-composite films is very sensitive to the presence of Pb ions. The films may be developed as excellent sensing films for Pb ions in water. CdSe/ZnS nanoparticles encapsulated within a silica shell and immobilized on the tip of an optical fiber by a polyvinyl alcohol (PVA) polymer coating was synthesized by Sung and Lo.¹⁴⁰ This high performance fiber optic sensor for Cu²⁺ ion is an ideal candidate for applications in chemical and medical detections. 8-Hydroxyquinolines (HQs) functionalized ZnS nanoparticles (NPs) with an amine-capping layer (ZnS–NH₂–Q NPs) were utilized by Feng et al.¹⁴¹ for facile and sensitive detection of TNT through fluorescent resonance energy transfer. Linear fluorescence intensity response for TNT was observed in the range of 0–1.89 μM and allows the quantitative detection TNT, with a detection limit down to 10 nM.

12.6 Hydrogen generation

Chang et al. investigated the abilities of photocatalytic H₂ production by Ni-doped ZnS–graphene composite photocatalysts.¹⁴² They discussed the effects of introducing graphene, doping, and decorated ZnS on the surface chemistry, crystalline property, optical property, surface morphology, and photocatalytic hydrogen production performance. Ni-doping and decorated ZnS on graphene improved the photocatalytic H₂ production activity because of improved dispersing property, increased surface area, increased absorption, and enhanced transfer of photogenerated electrons. Zhang et al.¹⁴³ reported the high solar photocatalytic H₂-production activity over the noble metal-free reduced graphene oxide (RGO)–Zn_xCd_1−xS nanocomposite prepared by a facile coprecipitation-hydrothermal reduction strategy. The work justifies that RGO is a promising substitute for noble metals in photocatalytic H₂-production. Cu₂S-incorporated ZnS nanocomposites for photocatalytic hydrogen evolution were reported by Michael et al.¹⁴⁴ Nanoflakes of ZnS–Cu₂S showed good results for visible light photocatalytic hydrogen production activities.

12.7 Drug delivery

ZnS–cellulose nanocomposites (ZS/CNC) were synthesized and explored for controlled drug delivery of ofloxacin by Pathania et al.¹⁴⁵ They found that maximum drug loading of 78% was observed at 2.2 pH. ZS/CNC showed antibacterial activity against E. coli bacteria.

Some other application of ZnS nanocomposites has been listed in Table 2.

Table 2 Other application of ZnS nanocomposites

Composition	Matrix	Synthesis method	Application	Ref.
ZnO:Mn/ZnS	ZnS	Seed-mediated growth	Diluted magnetic semiconductors	146
2,20-(Ethylenedioxy)-bis-ethylamine-modified ZnS:Mn/ZnS QDs	Folic acid-conjugated	Chemical method	Folate receptor-mediated delivery of folic acid-conjugated ZnS:Mn/ZnS QDs	147
ZnS	Graphene oxide (GO)	Chemical method	Fluorescence enhancement	95
ZnS	Mannosylated chitosan	Chemical method	Fluorescent probe	84
ZnS	Polyethyleneimine (PEI) hyperbranched polymer	Chemical method	Semiconductor material	148
ZnS:Mn	Folate conjugated carboxymethyl chitosan	Chemical method	Targeted drug delivery and imaging of cancer cells	149
ZnS:Mn, ZnS:Cu	Bleached Kraft fibres (Pinus radiata)	Chemical precipitation method	Hybrid material	99
ZnS	Ethyl cellulose	Oil-in-water nanoemulsions by a phase inversion process	Bioanalytical application	107
ZnS	PVA	Hydrothermal	—	150
ZnS	Graphene	Electrostatic self assembly method	Photocatalysis	151
InP/ZnS	Oxadiazole-carbazole copolymer (POC)		White light emission	152
ZnS	N,N′-Dimethylacrylamide	Innovatory method	High refractive index optical material	153
ZnS microspheres	g-C3N4	Precipitation route	Photocatalysis	154
ZnS	Fe3O4	Solvothermal	Bifunctional magnetic optical material	155
ZnS	Chitosan-g-poly(acrylamide)	Microwave radiation	Controlled drug delivery	156
ZnS:Cu nanocrystals	Sulphonated polystyrene methods	Ion exchange method	Light emitting diodes	157
ZnS	Graphene	In situ synthesis via non covalent functionalisation of graphene	Electrochemical sensor	158
ZnS-nanocomposites thin films	Poly-(dimethyl)-block-(phenyl)siloxane	Dip casting, spin coating	High-refractive index polymer media	159

---

13. Conclusion and futuristic outlook

The ability to control the nanoparticle diameter can be a challenging task for the application of these functional materials. Controlling the size and packing density of nanocrystals on a biological scaffold can be an effective way of tuning the different properties of the nanostructures. Therefore, it is important to understand the parameters that influence their formation.^160,161 From the authors point of view, the synthesis of polymer based ZnS nanocomposites is a promising area of research for designing novel functional hybrid materials due to their unique optical and electronic properties. There is a rapid increase in demand to synthesize novel semiconductor nanocrystals embedded/dispersed in polymer framework for a variety of practical applications. Polymer stabilized ZnS nanomaterials, in its variety of manifestations, has already been used successfully to generate interesting and useful materials for applications that involve plasmonics and medical and biological imaging. In spite of the fact that significant advances have been made in applying ZnS nanocomposites in diverse fields but still by careful selection of synthetic techniques and exploiting the unique chemistry of the polymeric nanocomposites, these materials can be designed, modified and fabricated for new interesting applications in other areas, such as labelling and tracking agents, drug delivery, optics, information storage, optoelectronics and magneto-optic applications etc.

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